This invention relates to the field of adaptive optic systems used to overcome blurring in images caused by atmospheric turbulence, and in particular, to a new phase sensor for adaptive optics systems.
An adaptive optics system automatically corrects for light distortions in the medium of transmission. For example, if you look far down a road on a very hot and sunny day, you will often see what is usually called a mirage. What you are seeing is the result of rapidly changing air temperature causing the air to act like a thick, constantly bending lens. As another example, the twinkling of stars is due to changes in the atmosphere surrounding the Earth. Although twinkling stars are pleasant to look at, the twinkling causes blurring on an image obtained through a telescope. An adaptive optics system measures the characteristics of the lens and corrects for atmospheric turbulence using a deformable mirror (DM) controlled by a computer. The device that measures the distortions in the incoming wavefront of light is called a wavefront sensor.
Light from a nominal point source above the atmosphere enters the primary aperture of an adaptive optics system and is split between a camera and a wavefront sensor employed therein (See FIG. 1). The sensor measures the wavefront distortion and controls (i) a tilt mirror to stabilize the image and (ii) a deformable mirror (DM) which restores the image sharpness lost to atmospheric turbulence. In recent years, the technology and practice of adaptive optics have become well knownin the astronomical community.
The most commonly used approach in a wavefront sensor is the Shack-Hartmann method. As shown in FIG. 2, this approach is completely geometric in nature and so it has no dependence on the coherence of the sensed optical beam. The incoming wavefront is broken into an array of spatial samples, called subapertures of the primary aperture, by a two dimensional array of lenslets. The subaperture sampled by each lenslet is brought to a focus at a known distance F behind each array. Because the lateral position of the focal spot depends on the local tilt of the incoming wavefront, a measurement of all the subaperture spot positions is a measure of the gradient of the incoming wavefront. A two-dimensional integration process called reconstruction can then be used to estimate the shape of the original wavefront, and from there, derive the correction signals for the deformable mirror.
Geometric sensors are more rugged and provide measurement accuracies that compare favorably with interferometric sensors. The geometric sensor divides the full aperture input wavefront into a number of subaperture images with an array of small diameter lenses. The subaperture images are focused as a two dimensional spot pattern onto a photodetector array which provides the X-Y phase gradient of each spot image as a representation of the average tip/tilt of each subaperture segment. Each segment phase gradient is converted to a phase estimate by a microprocessor-based reconstruction algorithm and the sum of the phase estimates provides a reconstruction of the wavefront""s full aperture phase profile. Measurement inaccuracies due to optical distortion or misalignment of the sensor""s optics are minimized by combining the received wavefront with an internal reference laser wavefront upstream of the subaperture optics and measuring subaperture tilt/tip as the difference in spot position between the two waves.
Since the reference wave suffers no atmospheric distortion, any displacement of the reference wave""s subaperture spot position from that of the subaperture""s chief ray is attributable to sensor distortion. The differential spot position between the two waves, therefore, provides an accurate measure of the received wavefront""s distortion. The geometric sensor is more tolerant of vibration and temperature conditions which, together with its simplicity, allows it to be used in a greater number of adaptive optic applications outside of the laboratory.
Briefly stated, a dispersed Hartmann sensor includes a Hartmann lenslet in combination with a dispersive element, whereby a Hartmann spot formed by light passing through the Hartman lenslet is dispersed parallel to the phase step of the light. The shape of the blur spot can then be examined at many wavelengths. Measuring the size of a discontinuity in the wavefront of light is then performed by forming a single image of the wavefront, dispersing the image in wavelength using a combination of a Hartman lenslet and a dispersive element, and analyzing the dispersed image along a dispersion direction of the dispersed image to measure the size of the discontinuity.
According to an embodiment of the invention, a dispersed Hartmann sensor includes a Hartmann lenslet in combination with a dispersive element, whereby a Hartman spot formed by light passing through the Hartmann lenslet is dispersed at a known angle to a phase step of the light.
According to an embodiment of the invention, a method for measuring the size of a discontinuity in a wavefront of light includes the steps of (a) forming a single image of the wavefront; (b) dispersing the image in wavelength using a combination of a Hartman lenslet and a dispersive element; and (c) analyzing the dispersed image along a dispersion direction of the dispersed image to measure the size of the discontinuity.
According to an embodiment of the invention, a mirror array includes a first layer having a plurality of mirror segments, each mirror segment consisting of a center portion and a surrounding non-center portion; a second layer having a plurality of Hartmann subapertures and a plurality of dispersed Hartmann subapertures; said Hartmann subapertures being arranged over said center portions of said plurality of mirror segments; and said dispersed Hartmann subapertures being arranged over those edges where said plurality of mirror segments join one another.
According to an embodiment of the invention, a system for measuring the size of a discontinuity in a wavefront of light includes means for forming a single image of said wavefront; means for dispersing said image in wavelength using a combination of a Hartman lenslet and a dispersive element; and means for analyzing said dispersed image along a dispersion direction of said dispersed image to measure the size of said discontinuity.